San Francisco could endure month's worth of rain in a single day this week

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A system diving down from the Northwest and tapping into a moisture plume over the Pacific Ocean is set to reach California Wednesday, delivering an unusual soaking to the San Francisco Bay Area in the middle of May.

The rainfall is forecast to start over the North Bay Wednesday afternoon, spreading across the central Bay Area that night and continuing into Thursday. Lingering showers are possible Friday and through the weekend.

"This is more typical of a winter-type system that we would see," says forecaster Roger Gass with the National Weather Service office in Monterey. "It's not unusual to get systems in the month of May, but the strength of the system is unusual. It looks to be a decent rain maker."

Gass adds the system is considered a weak atmospheric river because it's pulling moisture from the Pacific.

Rainfall totals will range from a .25 to .75 inch in urban areas to .75 to 1.25 inches in the North Bay and Santa Cruz mountains.

With this storm, the region could see more than the monthly rainfall total for May in a single day. Most spots in the region see less than an inch of rain on average in May.

"The bulk of the rain is forecast for Wednesday into Thursday," Gass says. "We've had dry conditions as of late, so we shouldn't have any major impacts. Most of the soils can absorb the storm runoff."

The rain is likely to impact the Wednesday evening commute in the North Bay and motorists across the region should be prepared for wet conditions Thursday morning.

Higher elevations could see strong southeasterly winds, but generally winds won't be a major factor with this system.

Leading up to the storm, Monday will be seasonably cool with morning fog, afternoon sunshine and dry conditions. Afternoon highs are expected to reach the high-60s on the coast and around the bay and the low-70s inland.

"Tuesday it starts to cool down across the Bay Area with highs in the 60s," says Gass.

Temperatures will dip even more on Wednesday ahead of the rain with afternoon highs across the region in the 50s.